1. Field of the Invention
The field of art to which the claimed invention pertains is solid-bed adsorptive separation. More specifically, the claimed invention relates to a process for the separation of ethylbenzene from a feed mixture comprising ethylbenzene and xylene isomers which process employs a solid adsorbent which selectively removes the xylene isomers from the feed mixture thereby producing a fluid raffinate stream comprising ethylbenzene.
2. Description of the Prior Art
It is well known in the separation art that certain crystalline aluminosilicates can be used to separate hydrocarbons species from mixtures thereof. In particular, the separation of normal paraffins from branched chained paraffins can be accomplished by using the type A zeolite which has pore openings from 3 to about 5 Angstroms. Such a separation process is disclosed for example in U.S. Pat. Nos. 2,985,589 and 3,201,491. These adsorbents allow a separation based on the physical size differences in the molecules by allowing the smaller or normal hydrocarbons to be passed into the cavities within the crystalline aluminosilicate adsorbent, while excluding the larger or branched chain molecules.
U.S. Pat. Nos. 3,265,750 and 3,510,423 for example disclose processes in which larger pore diameter zeolites such as the type X or type Y structured zeolites can be used to separate olefinic hydrocarbons.
In addition to separating hydrocarbon types, the type X or type Y zeolites have also been employed in processes to separate individual hydrocarbon isomers. In the process described in U.S. Patent 3,114,782 for example, a particular zeolite is used as an adsorbent to separate alkyl-trisubstituted benzene; and in U.S. Pat. No. 3,668,267 a particular zeolite is used to separate specific alkyl-substituted naphthalenes.
Because of the commercial importance of para-xylene, the more well-known and extensively used hydrocarbon isomer separation processes are those for separating para-xylene. Para-xylene is used in the manufacture of terephthalic acid which in turn is subsequently employed in the manufacture of various synthetic fibers such as Dacron which fiber is a trade-marked product of the duPont Company. In processes described in U.S. Pat. Nos. 3,558,730; 3,558,732; 3,626,020; 3,663,638; and 3,734,974 for example adsorbents comprising particular zeolites are used to separate para-xylene from feed mixtures comprising para-xylene and at least one other xylene isomer by selectively adsorbing para-xylene over the other xylene isomers. In such processes the adsorbents used are para-xylene selective; para-xylene is selectively adsorbed and recovered as an extract component while the rest of the xylenes and ethylbenzenes are all relatively unadsorbed with respect to para-xylene and are recovered as raffinate components. In contrast, the present invention relates to a process for the separation of ethylbenzene from a feed mixture comprising ethylbenzene and a plurality of xylene isomers in which essentially all of the xylene isomers are selectively adsorbed and recovered as extract components leaving only ethylbenzene relatively unadsorbed with is recovered in high-purity as a raffinate component. The adsorbent employed is "all-xylene" selective rather than para-xylene selective as are the adsorbents used in para-xylene separation process thus distinguishing my process from the prior art separation processes. Specifically, I have found that adsorbents comprising sodium-type Y zeolites having a SiO.sub.2 /Al.sub.2 O.sub.3 mole ratio greater than about 4.5 exhibit selectivity for all the xylene isomers with respect to ethylbenzene thereby making separation of ethylbenzene from xylene isomers by solid-bed selective adsorption possible.
Ethylbenzene is used as a raw material in the production of styrene monomer. Ethylbenzene can be and is commercially produced from the alkylation of benzene with ethylene. The cost of and competing demands for necessary benzene and ethylbenzene feed streams have, however, prompted new efforts to recover ethylbenzene from various C.sub.8 aromatic feed streams which already contain ethylbenzene. Such feed streams for instance, include C.sub.8 aromatic extracts produced by a typical solvent extraction process from a pyrolysis gasoline or from a naphtha which has been reformed with a platinum-halogen-containing catalyst. Additionally, C.sub.8 aromatic cuts of hydrogenated pyrolysis naphthas or reformates prepared by fractionation without solvent extraction contain varying amounts of ethylbenzene. The particular utility of the process of my invention is that it offers a method for recovering ethylbenzene from a feed stream which already contains ethylbenzene.
Ethylbenzene can, of course, be separated from the xylene isomers by fractionation but because its boiling point is within about 4.degree. F. of that of para-xylene, the fractionation can be achieved only with the more intricate super-fractionators. Typical ethylbenzene fractionators contain 300 to 400 actual trays and require about a 25-50 to 1 reflux to feed ratio. The process of my invention therefore offers a competitive alternative to the separation of ethylbenzene by super-fractionation.